1. Technical Field
The invention relates to mobile communications and, more particularly, to better supporting communication with mobile devices in motion at high speed.
2. Discussion of Related Art
The following discussion refers specifically to developments underway in the uplink (UL) part of the Third Generation Partnership Program (3GPP) Universal Terrestrial Radio Access Network (UTRAN) long term evolution (LTE) often referred as 3.9G but is not limited to that context.
In 3G LTE there is a need for a scheduling request (SR) channel for the uplink (UL) to be defined and, more specifically, a method is needed for SR transmission applicable for high User Equipment (UE) velocities. A scheduling request is used to indicate that the UE has some data to transmit towards the network side.
It has been agreed in the RAN1#47bis meeting in Sorrento that a non-contention based scheduling request (SR) mechanism for time synchronized users is to be supported.
FIG. 1 shows a transmission of an asynchronous scheduling request indicator message 1 from the UE to the base station where the UE does not yet have an uplink data assignment and a scheduling grant message 2 is shown being sent back. On the other hand, if the UE already has an uplink data assignment, it is in the stage 3 of FIG. 1 and new scheduling requests are transmitted in-band (Scheduling Request+Data).
A multiplexing scheme for the SR is presented in Document R1-072307, “Uplink Scheduling Request for LTE” 3GPP TSG RAN WG1#49, Kobe, Japan, May 7-11, 2007, Nokia Siemens Networks, Nokia, as shown in FIG. 2 hereof where we proposed the combination of block-spreading and CAZAC (Constant Amplitude Zero Autocorrelation) sequence modulation as a method to send the SR.
In Document R1-070379 from the 3GPP TSG RAN WG1 Meeting #47bis held in Sorrento, Italy, Jan. 15-19, 2007, two different ways of generating the SR were considered: a coherent multiplexing scheme and a non-coherent scheme. A coherent multiplexing scheme is similar to the structure that was agreed in Malta to be used for uplink ACK/NACK transmission (3GPP TSG RAN WG1 Meeting #48bis, St. Julian's, Malta, Mar. 26-30, 2007). However we prefer the non-coherent scheme for the SR because of better multiplexing capability. Furthermore, we considered a scheme where only a positive SR is transmitted (i.e., on-off keying).
Regarding to the UE velocity it has been stated in [TR 25.913] that                The E-UTRAN (Enhanced-UTRAN) shall support mobility across the cellular network and should be optimized for low mobile speed from 0 to 15 km/h.        Higher mobile speed between 15 and 120 km/h should be supported with high performance.        Mobility across the cellular network shall be maintained at speeds from 120 km/h to 350 km/h (or even up to 500 km/h depending on the frequency band). For the physical layer parameterization E-UTRAN should be able to maintain the connection up to 350 km/h, or even up to 500 km/h depending on the frequency band.        
It seems that the operation area of the highest UE speeds will play quite an important role when standardizing different functions of the LTE system (this was the case e.g., with RACH (Radom Access Channel)). Beside the fact that the highest UE velocities need to be supported, performance differences between various concepts are typically biggest in the extreme operation area, such as the highest UE velocities.
One of the requirements for SR is that it should support a high enough number of simultaneous UEs in order to keep the system overhead caused by SRs small enough. In order to maximize the multiplexing capacity with CAZAC sequence modulation, the spreading factor (SF) of block spreading code is maximized. The preferred SR multiplexing scheme is presented in FIG. 2. The number of parallel SR resources per slot equals to 12*7=84 in the illustrated scheme.
The multiplexing between the different user equipments is achieved through the code domain orthogonality. Cyclic shifts of Zadoff-Chu (ZC) sequences are used as the orthogonal codes. As shown in FIG. 2, the maximum number of orthogonal codes can be computed as 12*7=84. The orthogonality within a single block, or FDMA (Frequency Division Multiple Access) symbol, is limited by the channel delay spread and the sinc pulse shape used in the transceiver. Between the blocks the orthogonality is limited by the channel Doppler spread as well as the frequency error. In practice, the number of orthogonal codes can be less than 84 due to these phenomena.
It is noted that there is a problem caused by the SF=7, i.e., that different cyclic shifts of the same block level code start to interfere with each other as the UE speed increases. This means that it is difficult (or even impossible) to provide sufficient performance for 360 km/h case when using an on-off keying-based SR mechanism. This issue is demonstrated in FIG. 3, which shows the attenuation between different cyclic shifts of a certain block level code, for a given cyclic shift of frequency domain CAZAC code.
Related art technique would be to decrease the SF of block level spreading. We note that this approach will                Either decrease the multiplexing capacity quite dramatically (e.g., combination of SF=3 and SF=4 would mean that the multiplexing capacity is calculated according to SF=3)        Or decrease the SR coverage by TDM component        